The invention relates to a process and apparatus for conversion of an N-bit digital data word into an analog voltage value by comparing the digital data word with an analog reference voltage with a ramp-shaped time dependence.
Digital-to-analog conversions are performed in number of control applications and in telecommunication. The digital control of data conductors or lines of a liquid crystal display screen is a special field of application. The so-called "active-matrix liquid crystal display screens", also abbreviated as AM-LCDs, will in the future increasingly replace the conventional display screens having cathode ray tubes because of a number of advantages for television and data processing fields. The possibility of distortion-free production of an image, reduced control voltages, and, as a result of a reduced power requirement, a high resolution, the absence of X-radiation and strong magnetic fields in the outer field of the display screen and the possibility of manufacture in an economical technology can be added to the advantages of a flat and light structure that are inherent in the liquid crystal display screens. Liquid crystal display screens can also be used as light valves in a projector for video screens having a large scale format.
Liquid crystal display screens comprise a plurality of matrix-like image points, each image point being associated with a circuit element. Thin film transistors are frequently used as the circuit elements. The screen information is applied to column conductors and written line-wise into the image point memory by the circuit elements. Thus the n row conductors must be controlled so that only one line conductor of the entire n lines of the image point has a sufficiently high potential for 1/n-th of the image duration so that the image point capacitor can be charged at the data voltage corresponding to the image point information by the circuit element. During the balance of the image scan time, in which a line is not selected, the image point capacitor must not be discharged by the circuit element. This is accomplished by control of the line with a sufficiently reduced voltage.
Different methods are already known for control of the column conductors in a compatible technology for making a liquid crystal display screen. Both analog control circuits and also digital control circuits are used for that. The image point information is present in the digital control circuits, e.g., as gray values in digital code, and is converted by a digital/analog converter into an analog voltage value. A digital/analog conversion with the help of an analog voltage ramp (i.e. a voltage having a ramp-like time dependence), which is connected by switch elements to each of the column conductors of the display screen, is described in R. G. Stewart, et al.: 1990 SID Symposium Digest, pp. 319 to 322. The switch elements connecting the reference voltage having the ramp-like time dependence and the column conductors are first closed, i.e. the column voltages follow the ramp voltages. A digital counter, which was loaded at an earlier time with a data word containing the image point information of the concerned column, is incremented for each column synchronously with the analog ramp voltage. This data word then determines the time point at which the counter reaches its maximum value and thus the time point for opening of the concerned switch element. The column voltages remain after that at the ramp voltage values occuring at the opening of the switch elements.
A great advantage of the digital/analog conversion with the aid of an analog reference voltage having the ramp-like time dependence is that the conversion characteristic line, i.e. the correlation of the analog voltage values to each digital data words, is nearly arbitrarily selectable, since the time course of the ramp-like reference voltage is varied. The principle used by R. G. Stewart is generally connected with a very high circuit expense. Thus for each column N+i switch elements, a flip-flop and an N-bit counter are required. The flip-flops and the counters are particularly expensive.